Alpha + masking + clipping headache

Question

Hi,

Here's a graphical explanation of 1) what I have and 2) what I want

http://tinyurl.com/yg5zboy

My understanding is I need one or maybe two masks to accomplish this. As you can see, I want to combine both alpha channel (for the red semi circle) and a background transparency (to see the background shuttle, even through the hole in the mask). I can't figure out if I need grayscale PNG mask (with/without alpha?) or an image mask (how to create one from a PNG? what kind of PNG? 8bits, 24?). Well I tried multiple ways but none gave me the final result I described. Can somebody tell me how to do or point me to some example/code?

Thanks.

--G.